Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or a similar ceramic casserole dish.

Cook the pasta in a large pot of salted boiling water until just shy of al dente (about 1–2 minutes less than package instructions). Drain, rinse briefly with hot water, and set aside.

In a large saucepan over medium heat, melt 3 tablespoons of butter. Add the flour and whisk constantly for about 1–2 minutes to make a roux.

Slowly whisk in the milk (and broth if using) a little at a time, whisking out any lumps. Bring to a gentle simmer and cook for 4–6 minutes until the sauce thickens enough to coat the back of a spoon.

Remove the sauce from heat and whisk in the Dijon, garlic powder, onion powder, salt, and pepper.

Stir in the shredded cheddar and Gruyère (reserve a handful for topping) until smooth. If using, whisk in the Greek yogurt or sour cream.

Combine the cooked pasta and cheese sauce in the pot (or a large mixing bowl), folding gently to coat evenly. Stir in any add-ins.

Transfer the mixture to the prepared baking dish. Sprinkle the reserved cheese evenly over the top.

In a small bowl, mix panko breadcrumbs with 2 tablespoons melted butter and sprinkle over the top.

Bake uncovered for 20–25 minutes, until bubbly and golden on top. For an extra golden crust, broil for 1–2 minutes — watch closely to avoid burning.

Let the bake rest for 8–10 minutes before serving to set. Garnish with chopped parsley and serve warm.